If you're going to pick up the first book of a mammoth quadrology, you want to know that the time invested will be worth it in the end. Accordingly, this is a capsule review of the Otherland series as a whole, rather than this single work.
The short answer: it depends on if you like Tad Williams' narrative style so much that you can overlook the flaws. Fans of the author are likely to love Otherland. It contains well-drawn, generally sympathetic characters swept up in an epic story that draws on established cyperpunk themes, but with Williams' characteristic focus on the internal thoughts and feelings of the protagonists rather than on the action. The VR settings are interesting, the story is epic in both scope and length, and Williams does manage to bring most (not all) of the strands of the story together fairly convincingly in the last book.
The problems? First and foremost, the books are too long. The series could be chopped down by two-thirds without losing any of the important plot elements. Fans of cyberpunk as a genre, where the pacing is usually frenetic, will be exasperated by seemingly interminable scenes that, while prettily drawn, do little or nothing to advance the overall story.
Second, the characters are too static. By early in book two you've learned all you need to know about how most of the numerous central characters. While their relationships to one another evolve (albeit very slowly) the characters themselves reveal no new facets to surprise or entertain the reader. Renie is always lovably stubborn, Orlando always stoically perservering in spite his medical condition, etc. etc.
Third, the metaplot is unoriginal. While Williams has an interesting take on VR, the 'epic story' driving the books is essentially a rehash of the big themes in Neuromancer, William Gibson's seminal work from 1983. Told in a very different fashion, but the same basic ideas nonetheless.
Fourth, and most damning IMHO, the way the 'secrets' of Otherland are revealed can only be described as [bad]. Williams has serveral *thousand* pages to expose you to his imaginary world, yet when it all comes down to the cruch he resorts to the cheap expedient of having one of the major characters explain to all the other characters 'what is really going on' in a scene more appropriate to the climax of a 1940s murder mystery than an SF novel.
Having the characters discover the deepest secrets of Otherland piece by piece as they travel would have been fun. If Williams had borrowed another convention of mystery fiction and given the reader just enough clues to allow them to, if they were very clever, put it all together, then the tell-all speeches at the end would at least have the redeeming quality of letting you know if you'd guessed right. Sadly, Williams keeps key pieces of the backstory from you until the end, and the trip around Otherland is more of 'An Extended Tour of Virtual Reality as Imagined by Tad Williams' than it is plot exposition. I'm sure the author wanted to save some ideas to maximise their emotional impact, but, for me, the manner in which he finally reveals them made the Big Secrets seemed hackneyed and trite rather than "oh, wow!"
Fourth, the capture of the main villain of the piece was pure Hollywood, and I do not mean that as a complement. That part of the ending was so blatantly predictable (if you have watched any horror movies at all) and so obviously designed to set up a sequel, that I found myself wondering if Williams wrote it specifically to appeal to people who might want to buy the film rights. Blech.
Enough carping. All in all, for me Otherland was an entertaining, though not gripping, story. I enjoyed much of it, liked several of the characters enough to care about them, and although I was disappointed in the way the ending was handled, I have to give the author credit for keeping me interested through four very lengthy books. Is it good enough to read again? Not for me. Do I recommend it to others? As I said at the top, it depends on whether or not they like Tad Williams' books in general. If you do, go for it.